As has already been pointed out by oshalla, you don't want to use length on an array, but rather scalar. A few other important notes:

• Every time you my a variable, you get a fresh variable. This means that you (almost) never want to use two mys in the same scope. For example, if you typed

  my $a = 1;
  my $a;
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  then you would find that $a was uninitialised.
• qw// is a quoting operator that takes a string and splits it on whitespace, putting the result into an array. In our case, qw/1 2 3 4/ evaluates to the array (1, 2, 3, 4). The reason that they make you think of regular expressions is that /EXPR/ is short for m/EXPR/, the matching operator. The slashes (in qw//, m//, and other quote-like operators) can be changed to any of (), [], {}, or any repeated delimiter that you choose (like qw+1 2 3 4+)—although be careful if you use '' or ##.
• @array = (@array, $new_entry) is more idiomatically written as push(@array, $new_entry). See push.